A scintillator is a substance which, when hit by radiation such as α rays, β rays, γ rays, X-rays, or neutron rays, absorbs the radiation to emit fluorescence. The scintillator is used for radiation detection in combination with a photodetector, such as a photomultiplier tube, and has wide varieties of application fields such as the medical field, for example, tomography, the industrial field, for example, nondestructive inspection, the security field, for example, inspection of personal belongings, and the scientific field, for example, high energy physics.
Among the scintillators are various types of scintillators available according to the types of radiation and the purposes of use. They include inorganic crystals such as Bi4Ge3O12, Gd2SiO5:Ce, PbWO4, CsI, and KI, organic crystals such as anthracene, fluorescent substance-containing polymers such as polystyrene and polyvinyltoluene, and special scintillators including liquid scintillators and gas scintillators. When neutron rays are the object of detection, they are generally detected by utilization of a nuclear reaction which converts neutrons promptly into charged particles having energy, since neutrons have strong power to pass through a substance without performing any interaction in the substance.
Typical characteristics demanded of the scintillator include a large quantity of light (light emission intensity), high stopping power for radiation (detection efficiency), and fast decay of fluorescence (fast response). The scintillator designed to detect neutrons, in particular, needs to have the ability to discriminate between neutrons and the γ rays, because a radiation capture reaction tends to occur between neutrons and the absorbing substance, generating γ rays.
A 6Li glass scintillator has so far been used as a scintillator for neutron detection. However, its manufacturing process has been very difficult and thus expensive, and there has been a limit to its upsizing. A scintillator for neutron detection, comprising a fluoride crystal, on the other hand, is advantageous in that an upsized scintillator can be produced at a low cost. For example, a scintillator comprising a lithium barium fluoride crystal has been proposed. However, this scintillator has high sensitivity to γ rays, and its background noise attributed to γ rays has been great. Thus, there has been need to take a complicated technique in using it as a scintillator for neutron detection (see Non-Patent Document 1).
In connection with such problems, the inventors have attempted to apply cerium-doped LiCaAlF6 crystals as scintillators for neutron detection. In making these attempts, they have performed the evaluation, etc. of scintillation characteristics under simulated conditions involving ultraviolet excited light emission or α ray irradiation. These crystals cause light emission by mechanisms in two stages, namely, generation of α rays which are secondary radiation due to the reaction of incident neutrons with 6Li (primary mechanism), and light emission of ultraviolet rays at about 290 nm ascribed to the electron transition of Ce ions by the α rays (secondary mechanism). However, the evaluations made so far have not involved irradiation with neutron rays per se. This means that the primary mechanism has not been evaluated, namely, that the efficiency of α ray generation has not been evaluated at all. As seen from these facts, the true characteristics of the crystals as scintillators for neutron detection have not been evaluated. As a result, the optimal composition of the scintillator for neutron detection has not been specified yet (see Non-Patent Document 2). The 6Li content (to be described later) in the crystals used in the above-mentioned publicly known technologies has only been of the order of 0.73 atom/nm3, and the inventors themselves have confirmed that such a content results in the insufficient efficiency of neutron detection.
Thus, an ideal scintillator for neutron detection which fulfills all the characteristics is not existent at the present time. In the present invention, a material comprising a substance which absorbs the incident neutrons to emit fluorescence is called a scintillator for neutron detection.
Non-Patent Document 1: C. W. E. van Eijik et al., “LiBaF3, a thermal neutron scintillator with optimal n-gamma discrimination”, Nuclear Instruments and Methods in Physics Research A 374 (1996) 197-201.
Non-Patent Document 2: Kentaro Fukuda, Kenji Aoki, Akira Yoshikawa and Tsuguo Fukuda, Abstracts of Lectures at the 66th Congress of the Japan Society of Applied Physics, No. 1, 211 (2005)